Physiology Ch. 76: Thyroid Metabolic Hormones

Front 1

The majority of metabolically activated hormone secreted by the thyroid

Back 1

T3- Thyroxine

Front 2

Follicles are filled with what substance? What lines the follicles?

Back 2

Colloid
Cuboidal epithelium

Front 3

Major constituent of thyroid colloid

Back 3

Thyroglobulin: contains hormones

Front 4

Primary controller of thyroid secretion

Back 4

TSH from the anterior pituitary

Front 5

How are iodides absorbed

Back 5

Absorbed from the gi tract in the same manner as Cl-

Front 6

What is the fate of most iodide

Back 6

Excretion by the kidneys. Only 20% is used for thyroid hormone synthesis.

Front 7

Iodide trapping

Back 7

The pumping of iodide across the basal cell membrane of the thyroid cells. The rate of trapping is mostly influenced by TSH

Front 8

Where does synthesis of thyroglobulin occur

Back 8

The ER and Golgi synthesize and secrete the thyroglobulin into the follicles.

Front 9

What is the major constituent that binds iodide? Where is this stored?

Back 9

tyrosine binds iodine and is stored within thyroglobulin. Thus, thyroid hormones are synthesized within thyroglobulin.

Front 10

What is the first essential step in thyroid hormone synthesis? How does this occur?

Back 10

Conversion of iodide ions to oxidized iodine. This conversion is promoted by the peroxidase enzyme and its accompanying H2O2.

Front 11

Where is peroxidase located?

Back 11

At the apical membrane, where it can combine with the tyrosine within the thyroglobulin at the exact point at which it exits the follicular cell into the colloid.

Front 12

What happens if the thyroid peroxidase is blocked or absent

Back 12

Thyroid hormone synthesis falls to zero.

Front 13

Organification

Back 13

the binding Iodine with thyroglobulin

Front 14

Enzyme that causes rapid formation of iodine with tyrosine molecules

Back 14

iodinase

Front 15

the majority of iodine combines with tyrosine to form what? What occurs next?

Back 15

Diiodotyrosine, which then couples to form T4. Diiodotyrosine is formed from monoiodotyrosine first, and is then iodinated to the T2 form.

Front 16

Where is T3 and T4 stored? why is this important?

Back 16

T3 and T4 are stored within the thyroglobulin in the cell follicles. This storage allows a 2-3 month supply of thyroid hormones.

Front 17

How do thyroglobulin molecules containing hormones, enter the cell?

Back 17

Via Pinocytic vesicles from the apical cell membrane

Front 18

How are thyroid hormones released from the thyroglobulin?

Back 18

Lysosomes within the cell membrane fuse with the vesicles, and proteases digest the thyroglobulin molecules. The free hromones then diffuse through the basal cell membrane

Front 19

What happens to most of the iodinated tyrosine within the thyroglobulin?

Back 19

3/4 remains in the mono and di-iodotyrosine form. After protease digestion, the iodine is cleaved from the tyrosine by deiodinase enzyme and the iodine and tyrosine are recycled.

Front 20

In people who have congenitally absent deiodinase enzyme, what occurs?

Back 20

Iodine deficiency

Front 21

What hormone is mainly used by the tissues?

Back 21

T3

Front 22

What protein carries T4 and T3 throughout the blood

Back 22

Thyroxine binding globulin

Front 23

How does the affinities for plasma binding globulins affect the release of the hormones

Back 23

because both hormones have very high affinities for the plasma proteins, they are released into the cell slowly. T4 is released over 6 days, while T3 is released over a day.

Front 24

What causes the long latent period and prolonged period of action of these hormones?

Back 24

The strong affinity for binding proteins in the plasma and within the cell. Also, is caused by the fact that thyroid hormones transcription and translation of proteins

Front 25

The nuclear thyroid hormone receptors have an extremely high affinity for what form?

Back 25

T3

Front 26

The thyroid hormone receptor within the nucleus is in what form

Back 26

the receptor forms a heterodimer with the retinoid X receptor at specific thyroid hormone response elements on the DNA.

Front 27

What effect does thyroid hormone have on mitochondria?

Back 27

Increases the number and activity of mitochondria, which increases the rate of ATP produced.

Front 28

What major enzyme is influenced by thyroid hormone? What is its effect?

Back 28

the Na-K ATPase. This in turn increases the rate of transport of ions through the cell membrane. This uses a large amount of energy and increases the amount of heat produced in the body.

Front 29

The affects of thyroid on growth occurs at what age

Back 29

the effects are seen mainly in growing children, in those who are hypothyroid, the rate of growth is greatly retarded, however, during hyperthyroidism the bones grow in length faster, while the epiphyses close at an earlier age. This may lead to overall shorter stature

Front 30

How does hypothyroidism affect cholesterol and triglycerides?

Back 30

Increases the cholesterol and triglyceride levels and leads to deposition of fat in the liver. In hypothyroidism there are reduced numbers of LDL receptors on the liver cells and cholesterol cannot be absorbed into the liver to be secreted in the bile and lossed in the feces.

Front 31

How does thyroid hormone affect fat and carbohydrate metabolism?

Back 31

Thyroid hormone increased lipolysis from adipose tissue and the free fatty acid concentration increases. It also increases the oxidation of free fatty acids.
It also stimulates all aspects of carbohydrate metabolism including increased insulin secretion with rapid uptake of glocuse into the cells, enhanced glycolysis, enhanced gluconeogenesis, and increased rate of GI absorption.

Front 32

How does hyperthyroidism affect vitamin requirements?

Back 32

Increases the daily need for vitamins

Front 33

How does hyperthyroidism affect cardiovascular output and respirations?

Back 33

Increased metabolism in the tissues causes rapid utilization of oxygen and increases the metabolic products formed. This causes vasodilation and the HR, strength, and CO increases to supply the metabolic demand.

Front 34

What fatal condition may occur during severe hyperthyroidism?

Back 34

The catabolism of proteins exceeds the production and the cardiac strength decreases, while the CO must remain high, this leads to cardiac decompensation and CHF.

Front 35

The rate of blood flow in hyperthyroidism is especially increased where?

Back 35

The skin, which helps to remove heat

Front 36

What effect can a clinician used to directly determine hyperthyroidism?

Back 36

thyroid hormone directly stimulates the cardiac muscle causing increased excitiation and an increased HR

Front 37

Explain the effects of thyroid hormones on the mean arterial pressure

Back 37

During hyperthyroidism the mean arterial pressure stays the same, however, the pulse pressure increases markedly because of the increased blood flow through the tissues between heartbeats. This causes an increase in systolic and a decrease in diastolic pressure.

Front 38

What are the effects of thyroid hormone on muscle

Back 38

thyroid hormone stimulates muscle contraction, however, hyperthyroidism leads to muscle weakness due to increased protein catabolism and hypothyroidism causes the muscles to become sluggish and slow to relax.

Front 39

What type of tremor is associated with hyperthyroidism?

Back 39

Fine muscle tremor that is distinguished from the coarse tremor of parkinsons. This is caused by increased neuronal synapse activity and is an important indicator of thyroid hormones affects on the CNS

Front 40

How does hyperthyroidism affect sleep

Back 40

The person is tired but unable to sleep

Front 41

How does thyroid hormone affect glucocorticoids?

Back 41

Increases the rate of deactivation of glucocorticoids in the liver. This leads to feedback increase in ACTH from the anterior pituitary and an increase in glucocorticoid secretion.

Front 42

How do thyroid hormones affect sexual functions

Back 42

results from a combination of direct metabolic effects and excitatory/inhibitory feedback from the ant pituitary.

Front 43

Effects of TSH on the thyroid gland

Back 43

1. increases the proteolysis of thyroglobulin and decreases the follicular substance.
2. Increased activity of the iodide pump.Increasing the intracellular to extracellular ratio
3. Increased iodination of tyrosine
4. Increased size and secretory activity of thyroid cells
5. Increased number of thyroid cells, with a change from cuboidal to columnar and infolding of the epi into follicles.

Front 44

The most important early effect after administration of TSH

Back 44

Initiate proteolysis of the thyroglobulin

Front 45

TSH uses what second messenger

Back 45

cAMP via activation of adenylyl cyclase, which activates protein kinases.

Front 46

TRH is secreted where? How does it reach the ant pituitary?

Back 46

TRH is secreted from nerve endings in the median eminence of the hypothalamus. From there it is transported by way of the hypothalamic-hypophysial portal blood.

Front 47

TRH exerts its influence on TSH via what second messenger?

Back 47

PLC

Front 48

What is the best known effect for increasing the rate of TRH and TSH secretion?

Back 48

Exposure to cold. This is from excitiation of the hypothalamus temperature control centers

Front 49

How does anxiety and stress affect thyroid hormone production

Back 49

Increased stress leads to increased sympathetic stimulation which causes a decrease in TSH due to the effects of the sympathetic nervous system to increase the metabolic rate and body heat. These effects are mediated by the hypothalamus.

Front 50

How does an increase in thyroid hormone affect TSH

Back 50

It directly decreases the rate of TSH secretion. This is independent of the hypothalamus.

Front 51

High concentration of this can lead to competitive inhibition of iodide transport into the cell. What long term affect may occur as a result?

Back 51

thiocyanates, perchlorate, and nitrate ions. These ions disrupt the formation of thyroid hormones, leading to an increase in TSH, which causes overgrowth of the thyroid colloid leading to goiter.

Front 52

Mechanism of propylthiouracil

Back 52

Decreases thyroid hormone by blocking the peroxidase enzyme and the coupling of iodinated tyrosine molecules. This may also cause a goiter.

Front 53

What happens during high concentrations of iodide molecules

Back 53

The normal endocytosis of colloid from the follicles is paralyzed and thus thyroid hormones are unable to be released. this leads to immediate shutdown of thyroid hormone secretion. However, they also decrease all phases of thyroid activity and the thyroid gland decreases in size and blood flow.

Front 54

Is the thyroid gland increased in size during hyperthyroidism? What is the TSH level? How does this occur?

Back 54

The thyroid gland is increased in size, but the TSH level is decreased. Usually anti-bodies called thyroid stimulating immunoglobulins bind to TSH receptors and cause continual activation of cAMP.

Front 55

What causes exopthalmia in hyperthyroidism?

Back 55

Edematous swelling of the retro-orbital tissues and degenerative changes in the extraocular muscles from anibody binding.

Front 56

How do you treat hyperthyroidism?

Back 56

The pt first receives propiothiouracil to reduce the Basal metablic rate. Next, iodides are given to shrink and reduce the thyroids blood supply. Finally, the thyroid is surgically removed.

Front 57

What is the most common cause of hypothyroidism?

Back 57

Autoimmunity that destroys the gland. This is first seen as thyroiditis, that leads to degeneration and fibrosis of the gland leading to decreased thyroid secretion.

Front 58

How does goiter occur in people without iodine deficiency?

Back 58

In pts with autoimmune disease thyroiditis may occur, leading to depressed levels of thyroid hormone and increased TSH. The increased TSH causes progressive growth of the thyroid colloid tissue

Front 59

Common abnormalities in the enzyme system for thyroid hormone formation

Back 59

1. Deficient iodide-trapping mechanism
2. Deficient peroxidase system
3. Deficient coupling mechanism
4. Deficient deiodinase enzyme

Front 60

How does myxedema occur? What type of edema is it?

Back 60

Greatly increased quantities of hyaluronic acid and chondroitin sulfate bound with protein form excessive tissue gel in the interstitial spaces and cause the interstitial fluid to increase.
This type is non-pitting edema

Front 61

Extreme hypothyroidism fetal life, infancy, or childhood may cause what?

Back 61

Cretinism